1. Field of the Invention
The present invention relates to a driving circuit for driving brushless DC motors.
2. Description of the Prior Art
In recent years, brushless DC motors which have their excited armature phases switched in turn by semi-conductor devices (such as transistors) according to signals from rotor position detectors (such as Hall effect devices) have been used in audio and video equipment. Usually, a brushless DC motor needs one rotor position detector and one armature excitation control circuit for exciting each phase. Accordingly, as the number of the phases of the motor increases, complexity of the control circuits increases. This would make the motor expensive and bulky. Hence, this scheme is not put into practical use. Therefore, in general, armatures of two or three phases are adopted. This has normally resulted in torque ripples ranging from 15 to 20%.sub.p-p, increasing variations in the motor speed. As a result, the wow or flutter of the audio or video equipment using such a motor increases.
Brushless DC motors recently available frequently make use of Hall effect devices as rotor position detectors. As is well known, Hall effect devices widely vary in sensitivity. For this reason, various circuit techniques have been employed to accommodate such sensitivity variations. Further, many attempts have been made to reduce torque ripples of brushless DC motors.
Japanese Laid-Open Pat. No. 59-35585 (hereinafter referred to as "reference 1") discloses a typical technique that uses three Hall effect devices as rotor position detectors. The scheme of the driving circuit disclosed in reference 1 is not described in detail herein, but, the operation of the driving circuit is characterized in that the coils are switched at such timings that the signals from the Hall effect devices become zero so as to be less affected by the sensitivity variations among the devices. Further, the operation is characterized in that the electric currents supplied to the coils are partially modulated in response to a signal synchronized with the rotation of the motor for canceling out the torque ripples.
The driving apparatus disclosed in the aforementioned reference 1 operates based on the timings at which the signals from the Hall effect devices become zero. This offers the advantage that the driving circuit is insusceptible to the sensitivity variations among the Hall effect devices. However, since the armature coils are alternately energized and deenergized, filter circuits containing relatively large capacitors for reducing the spike-like voltage produced by the switching of the coils need to be provided at the power supply terminals to the coils. Also, the driving circuit has the disadvantage that it is likely to produce vibration and noise, because the electric currents flowing into the coils are suddenly caused to cease and commence.